THE ALGM 149 



as gaseous, by their whole surface. Both the carbon- 

 dioxide which they require for assimilation, and the 

 oxygen necessary for their breathing, are present in 

 an absorbed state in the water, which at 'the same 

 time contains salts in solution quite sufficient to supply 

 the needs of these plants. 



The structure of an ordinary vegetative cell of- 

 (Edogonium is as follows : Within the cellulose wall the 

 protoplasm forms a hollow sac the primordial utricle 

 enclosing a large vacuole. The body containing the 

 chlorophyll is very peculiar. In most plants the 

 chloroplasts are small granules, numerous in each cell 

 (though Selaginella forms an exception to this). In 

 (Edogonium, however, there is only a single, very 

 large chloroplast in each cell. It lies in the primordial 

 utricle, and extends all round the cell, having the form 

 of a hollow cylindrical network. It is so large as to 

 give a green colour to the whole cell, when seen under 

 low powers of the microscope. Within the chloroplast 

 are several proteid granules (the pyrenoids), around which 

 starch-grains are deposited as a result of assimilation 

 in sunlight. Each cell contains a single large nucleus 

 embedded in the protoplasm which lines the wall. 



(Edogonium has no apical growing-point. In some 

 species (such as that figured) the end cell grows out into a 

 long hair, and takes no further part in the divisions. All 

 the cells of the filament, lying between the radical cell 

 and the terminal hair, divide by transverse walls, as long 

 as growth goes on. The formation of overlapping caps 

 on the cell-wall, at the upper end of some of the cells, is 

 due to the fact that after each division the wall of the 

 mother- cell splits near the top, and a new piece of cell- 

 wall is inserted between the broken edges as the 



